The mechanisms by which Salmonella typhimurium invade animal cells is being investigated with the HeLa cell model. An early phase of the invasion is the attachment of the bacteria to the outer surface of the HeLa cell membrane. The bacterial factors that contribute to adhesion are several but the mechanism by which the bacteria adhere appears to be caused by a protein on the surface of the bacterium. Adhesion, is not mediated by the common type 1 fimbriae however, but by a distinct surface component which binds to the HeLa cell membrane in the cellfree state and can inhibit the attachment of bacteria. Future studies include the isolation and chemical analysis of the salmonella adhesin and it's morphological characterization. The ability of mutants lacking the adhesin will be examined and the virulence of such mutants studied in an attempt to measure the ecological significance of the adhesive factor.